The binders used for a magnetic recording medium generally include vinyl chloride type binders such as a vinyl chloride/vinyl acetate copolymer, a vinyl chloride/propionic acid copolymer, a vinyl chloride/vinyl acetate/vinyl alcohol copolymer, and the like. Among those binders, a vinyl chloride/vinyl acetate/vinyl alcohol copolymer is widely used because hydroxyl groups of the vinyl alcohol units provide good dispersibility of ferromagnetic particles and the active hydrogen atoms of the hydroxyl groups react with isocyanate compound to form a crosslinked structure, thereby increasing the mechanical strength of the coated film.
In recent years, because the specific surface area of the magnetic particles has been increased as a result of making the magnetic particles finer and the aggregating force of the magnetic particles has been increased as a result of making the coercive force higher, the above described binders have not been able to provide satisfactory dispersibility and surface properties of the coated film. Thus, it has become difficult to increase the packing density of the ferromagnetic particles. Also, the durability, the magnetic characteristics and the electromagnetic properties of the magnetic recording medium are also insufficient.
It has been proposed to use, for example, a surface active agent as a dispersing agent. However, with respect to the surface active agent used, problems have arisen with the mechanical strength and the durability of the medium because since the surface active agent is a low molecular weight compound, powder drop out and blooming have been caused with the passage of time.
Under the circumstances described above, binders capable of improving the above properties of magnetic recording medium have been desired. It has been proposed to use a vinyl chloride/vinyl acetate copolymer or a vinyl chloride/vinyl acetate/vinyl alcohol copolymer as a binder in which a sulfonic acid group or its alkaline metal salt, a sulfuric acid group or its alkaline metal salt, a carboxyl group or its metallic salt, and the like is introduced into the side chain of the copolymers.
These binders can improve the dispersibility to some extent when compared with the conventional binders having no polar group, but even this improvement is insufficient for magnetic particles having extremely fine particle size and those having higher coercive force.
In recent years, magnetic recording mediums have been required to have high image and audio quality. In order to realize high image quality, it is necessary to closely contact the surface of a magnetic layer with a video head or an audio head. Therefore, it is important not only to improve surface smoothness of a magnetic recording medium but also to improve the dispersibility of ferromagnetic particles.
On the other hand, as the smoothness of a magnetic layer is increased, the friction on the running systems of a video tape recorder increases, thereby increasing static charge and the tension of the running tape. Under those harsh conditions, sufficient running durability has become much more necessary for magnetic recording mediums.
Hithertofore, a magnetic recording medium which is provided with a satisfactory surface smoothness of the magnetic layer, good dispersibility of ferromagnetic particles, and good running durability could not be obtained by a conventional magnetic recording medium.